KVM: selftests: Add a read-only mprotect() phase to mmu_stress_test

KVM: selftests: Add a read-only mprotect() phase to mmu_stress_test

Add a third phase of mmu_stress_test to verify that mprotect()ing guest
memory to make it read-only doesn't cause explosions, e.g. to verify KVM
correctly handles the resulting mmu_notifier invalidations.

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>